Method of and apparatus for receiving radio signals



Sept. 20, 1949.

R. T. KlLLMAN ET AL METHOD OF AND APPARATUS FOR RECEIVING RADIO SIGNALSFiled Dec. 3, 1945 12 AMP.

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Patented Sept. 20, 1949 UNITE o, PATENT.

METHOD: OF' AND APPARATUS FOR RECEIVING RADIO SIGNALS Robert'11.,Killmanand Frederick W..Beeslcy,.

Nashville, Tenn,

Application December 3, 1943,'ISerial=No.512,6941;

2 Claims- 1;

This" invention: relates to impmvementswin methods and apparatus forselectingand detecting radio waves;

In all; previous: methods ofyselecting a'desired signal from the manypresent in :the ether the signals, including; the one desired, .areusually picked up ;by an antenna and passed-ithrough one ormore-resonant circuits, composedof suitable inductance a-nd'capacitywhich aretuned to the frequency-of the desired signal. These resonantcircuits areusually parallelsresonant, and serve to increase the voltageappearin -across them at the frequency ofthe desired signal while thevolt;- ageat another frequencies isnotincreased; or:at least notincreased to such. extent as ,is the'voltage-at the resonant frequency;These resonant circuits are-usually coupled by electronic tubeamplifiers; and due to the selective and: amplifying action of such asystem the desiredisignalmay be picked out of the many signalsiimpingingon the-antenna andamplified -to the excl'usion of the unwanted signals.

Various, modifications have beenvmade' in the above described basicsystem, suchras for rexample, the; use of the. supereheterodyneprinciple wherein-the frequency of. the:.=-desi-red signal-is changed to:some arbitrary fixed intermediate frequency and then is passed throughresonant circuits. and amplifiers as described abovel- However all"previously known-methods of selecting. the desired signal from theWelter of signals present in the ether rely on the resonant circuit.

This type of signal selecting system has several disadvantages. As iswell, known bythose skilled in the art, in order to increase theselectivityof aresonant circuit it is necessary to decreasejheresistance-of the circuit, or in other words, to in.- crease the Q. Thishas the. effect iofrdeoreasing the decrement of the circuit and ifcarried-sumciently-far the decrement of the circuit becomes so smallthat so called tails? are formed onthe ends of the dots and dashes ofcodelsignals and the, higher modulation frequencies ofa. modulatedsignal are distorted vand attenuated since. the deer rement. of the,signalis. less than that of thefIeSO-r nant circuit and therefore thecurrents ,-in rthe resonant circuit cannot vary in amplitude asrap; idlyas the signal.

A further disadvantage of resonant circuits. as signal, selectors liesin the fact that they arecar pahle of being shock excited. A strayimpulse oirsuch; short duration that it; couldnot beheard mayshockthe;reso11am; circuit; so-that itgenens 1,-

ordinaryreceivers so; garbles; on,jams-.;

ates adamped wavertrainiofas fic nt dnrationitv cause-an-:.audibleimpulse.

Accordin ly it is; amobi'ect; of this inventlorsiio disclos s methodofnselectine. acles edsienaitfmm a number oi undesired-si nals: ofzdiiferent flier quencies and to amplify and demodulate thre desired,signal, and to partially or totallyexclnde: the undesiredsignalswithout, makin useof tuned; .Or resonant circuits; orthejmeqnivalent:.:

A further object otthis inventiomis to provide means, w r yasired-signal: e selected and. differentiated from an;ndesiredizsignalifor si n ls of: the ame:frequencit:provided-thereasiredrandundesired s enalsa enotrimphasei-theresults beingmostsatisfactory when:thetwo sig; nals are ofoppositephaser;

A further object of this invention is. toiprovlde means whereby theefiect; of- -atmosnheric 013218031 strays ;01 static upongtheedesiredusignalt ma n-he materially reducede ifrleql ncy ut.- ofe ppsite phase are transmitted simultaneously; one; of; said. waves beingmodue latedby the :desi-redsv intelligenceswhile-tthe rather wave ismodula-ted by an unintelligible; ffj -ing? signal which when bothtswavesare receiyedaby themrdesired intelligence .modulation tha-tgi lS1j,-1lIil ead;-;- able, but which whensreceived by; the receiving system ofthis invention canrabe separated: into its, twocomponents and thedesiredintellieehce becomesreadablei The means; by which theseandlothercbjects, as

will presently-appear; is accomnlished maywbe more. "readily;understoodi by"; efc rencemtoi the accompanying drawings; whereiniFig-glis a single channel system for receiving radiolsignalsemalr i -useof,-the.i rincip esi tithis; inventiom-Eig. 2 s a two channl-isystemzmakinguse ofmeutralizm tion-toeliminate a ls but :thedesiredsignal; and Fig'ure fi man-al ernate single channel system for receivingradi sis alsxmakin zusezof the:princivv ples, of this invention-.;;

Referrin naw; to-Fi 1n the. numeral ifl denotes theusual. rec iving:antennaspreferablyneriadic or untuned. The signals received byantenna-1E0 are passedthrough; and -,alnnlifi ed by thenntuned radieereqne yzamp ifier:l [1.1.

The ,outnut; of amplifier l t isapfllied acrossthe grid-J 3-.andthcathode I 6 ofiitheel'ectronili$11138 I12. An. adiustablee square iiwave generator... l8

' GGGSF z- HMQHE hrou h @theaadiustable when:

shifter l9 to the grid l5 and cathode it of tube l2. The square waveoutput of the generator [8 is of such voltage that when its positivepeak is applied to grid tube 12 operates as a class A amplifier but whenthe negative peak of the square wave voltage is applied to grid 15 tubeI2 is completely blocked and is incapable of passing signals receivedupon grid [3.

A plate or anode I1 serves to receive the electron stream emanating fromthe cathode l6 and acted upon by the two grids l3 and I5 and also feedsthe output of the tube l2 through an audio amplifier to a soundreproducer 2|.

The operation of the system just described is as follows: The desiredradio frequency wave emanating from a transmitter (not shown) togetherwith the many undesired waves impinge upon the antenna I0 and areamplified by th amplifier ll and passed on to grid l3. During the timeinterval that the desired radio frequency signal is passing from zerothrough its positive peak and back to zero on grid 13, the square wavegenerator l8, which is set to generate a square wave of a frequencyidentical to that of the desired signal, is adjusted to apply thepositive peak of its square wave output to grid 15 so that, during thisinterval, the tube l2 acts as a class A amplifier and the plate currentof the tube is a replica of the grid Voltage on grid 13, and the signalis passed on to the audio amplifier 20 and reproducer 2|. However duringthe time interval in which the desired radio-frequency wave passesfrom'zero through its negative peak and back to zero, the negative peakof the square wave output of generator I8 is applied to grid l5 so thattube I2 is blocked and no signal is passed on to the audio amplifier 20and reproducer 2|.

Thus it will be seen that only the positive half cycles of the desiredsignal are passed on to the audio amplifier 20. The envelope of thesepositive half cycles constitutes the audio modulation and this audiowave or envelope is amplified by the audio amplifier 28 and reproducedinto sound by the reproducer 2 I.

From the above it will be seen that the action is very similar toordinary demodulation except that the only signal which is completelydemodulated is the one of the frequency and phase of the square wavegenerator.

Any wave of a different frequency is not completely demodulated i. e.parts of both its negative and positive half cycles appear in the outputand cancel each other.

It will be noted that, while in the above description it was said thatthe positive half cycles of the desired signal were passed but thenegative half cycles were not, it might just as well be the other wayround, i. e., the negative half cycles could be passed through and thepositive half cycles blocked.

In order to make the system responsive only to positive half-cycles ofthe desired signal, a negative voltage may be introduced by the battery22, as shown in Figure 3 of such magnitude as to just cause platecurrent cut-off. In this condition grid I3 will be responsive only topositive half -cycles, while the negative half-cycles applied theretoserve only to drive grid l3 further negative.

By adjusting the phase shifter l9 so that during the interval thatpositive half cycles of the signal are applied to grid I 3 (the tube I2being biased to cut-off by the voltage above referred to inserted in itsgrid circuit by battery 22, as shown in Figure 3) so that the negativepeaks of the square wave voltage are applied to grid l5, thus blockingthe tube during this interval, the signal may be eliminated, while anyother signal having positive half-cycles which appear between thepositive half-cycles of the eliminated signal will appear in the output.

This phenomena allows a second circuit arrangement to be set up as shownin Fig. 2. In this figure the numeral l0 denotes the antenna, theuntuned radio-frequency amplifier H, tube l2, grid [3, grid l5, cathodeI6, plate IT, square wave generator l8, phase shifter [9, audioamplifier 20 and. reproducer 2| being the same as previously describedin reference to Fig. 1. The circuit through tube I2 is called channel A.

However a second channel, marked channel 13, between the amplifier IIand the amplifier 20, comprises the detector 24 and the phase inverter25. The detector 24 is biased to cut-off by the battery 23 and the tubeI2 is likewise biased to cut-ofi by battery 22. The connections,polarities, and voltages are so adjusted that the output of amplifier IIis divided, half going through channel A and half going through channelB.

The phase inverter 25 inverts the phase of the currents in channel B sothat they oppose those in channel A and any signal which appears both inchannels A and B will be completely neutralized and will not appear inthe combined output of channels A and B and will not be passed on toaudio amplifier 28 nor reproduced by the reproducer 2 l, while anysignal appearing in only one channel will not be neutralized andtherefore will be passed on to audio amplifier 2c and will be reproducedby reproducer 2 i.

In operation of the circuit shown in Fig. 2, the desired signal togetherwith many undesired signals, strays, static, etc., impinge upon antenna10 and are amplified by amplifier H and passed on to channels A and B inequal proportions.

Since the tubes in both channels are biased to cut-off only positivevoltages are effective in either channnel. In channel A the desiredsignal is eliminated by means of the action of the square wave generatorin blocking tube [2 during the intervals that the positive half cyclesof the desired signal appear on grid l3, but at all times between theseintervals the tube is responsive to any other positive voltages fromother signals or static and the envelope of these signals will be passedon as an audio signal to the amplifier 20. However the envelope of thedesired signal will not be passed since each positive half cycle of thedesired signal was eliminated as above described and all negative halfcycles have been eliminated due to the high bias voltage 22.

However the positive half cycles of the desired signal together with thepositive components of all the undesired signals and static etc. arepassed by channel B and the envelope of these positive impulsesconstitute an audio wave which is passed through the phase inverter 25which causes its phase to be opposite to the audio signal coming throughchannel A. These two audio voltages are combined and as pointed outabove all portions which are similar cancel out and only the desiredsignal which appears in only channel B is amplified by amplifier 2B andreproduced by reproducer 2i.

It is of course understood that the devices shown in Figs. 1 and 2include all the necessary and usual accessories for proper operationsuch as power and voltage supplies, shielding, filters,

networks, etc. some of which, such as the shield grid I4 and the filtercondenser 20a, are shown,

.tive half cycles of the oscillator output,

but others are not shown, these details and auxiliaries forming no partof the basic concept of this invention. It is also understood that thisinvention is not limited to the actual devices shown, but equivalentsfor any of the devices shown for the purpose used herein may besubstituted therefor. For example, for the square wave generator I8shown, there may be substituted a simple radio frequency oscillatortogether with a half wave rectifier which removes the posi- The negativehalf cycles which remain, when applied to grid 15, if of sufficientamplitude, have the same effect thereon as did the square Wave output ofgenerator l8, since the tube l2 will be blocked very early during eachof these negative half cycles and will remain so until very late in saidnegative half' cycle and it will not matter that the negative half cyclehas a much greater negative amplitude between these two points than isrequired to block the tube. 7

Also a properly wound neutralizing transformer or other means may beused to differentially couple channels A and B to amplifier of Fig. 2instead of making use of the phase inverter to cause the signals inchannel A to neutralize like signals in channel B.

While there has been described what are at present considered thepreferred embodiments of the invention, it will be obvious to thoseskilled in the art that various changes and modifications maybe madetherein without departing from the spirit of the invention, and it istherefore, aimed in the appended claims to cover all such changes andmodifications as fall within the true spirit and scope of the invention.

What is claimed is:

1. In a radio receiving system, a signal wave collecting system'which isnon-difierentiative as between desired and undesired signal waves, a twochannel signal wave passing system, each of said channels being normallyresponsive only to signal wave half cycles of one polarity, one of saidchannels being capable of being intermittently blocked so that duringthe blocked interval said channel is not responsive to signal waves ofeither polarity, means for blocking said last named channel during theinterval that the half cycles of the desired signal wave of the polarityto which said channel is normally responsive are applied thereto butallowing said channel to pass all signal waves of the polarity to whichit is normally responsive which are applied thereto during the intervalthat half cycles of the desired signal of the opposite polarity to thatto which the said channel is normally responsive are applied thereto,means for differentially combining the outputs of the two channels sothat all signal waves appearing in both channels are cancelled cut,while signal waves apearing in only one channel are not cancelled, andmeans for I translating the uncancelled waves into sensible phenomena.

2. In a radio receiving system, a signal collecting antenna, an untunedradio-frequency amplifier coupled to the antenna, two signal passingchannels A and B coupled to the amplifier, channel A comprising anelectronic tube having at least an anode, a cathode and two grids, thefirst grid being connected to the amplifier, means for generating anoscillatory voltage of rectangular wave form, means for adjusting thefrequency of said voltage, means for adjusting the phase of said voltagewith respect to time, means for applying the voltage to the second gridof the tube in such manner that during the interval that the positivehalf-cycle of said voltage is applied thereto the tube Will operate as aclas B amplifier and will be responsive only to the positive half-cyclesof signals applied to the first grid, but during the interval that thenegative half-cycle of the locally generated oscillatory voltage isapplied to the second grid the tube will be completely blocked and notresponsive to signals of either polarity applied to the first grid,channel B comprising a detector or rectifier system responsive to signalhalf cycles of only one polarity and means for differentially combiningthe outputs of channels A and B so that signals appearing in bothchannels will be cancelled while a signal appearing in only one channelwill not be cancelled, an audio amplifier for amplifying the uncancelledsignal, and an audio reproducer for translating said signal into sound.

ROBERT T. KILLMAN.

FREDERICK W. BEESLEY.

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